|Polyrhythm (Greek, â€˜many rhythmsâ€™) is the simultaneous use of contrasting rhythmic characteristics by two or more instruments or voices. There is some disagreement between the terminologies used for the discussion of rhythm in Western art music and those used to discuss the rhythmic, often percussive music of other cultures, such as African drumming; debate centres on the precise meanings of â€˜rhythmâ€™, â€˜metreâ€™, and â€˜pulseâ€™ or â€˜beatâ€™.
The most common, albeit diverse, definitions of polyrhythm are: (a) â€˜rhythmic counterpointâ€™, where simultaneously-performed parts use contrasting rhythmic patterns; (b) â€˜rhythmic canonâ€™, where simultaneously-performed parts have identical or closely similar rhythmic patterns overlapping or displaced in time; (c) where simultaneously-performed parts have different divisors of the same beat: for example when one part with beats divided into three units is set against another divided into four; (d) â€˜cross-rhythmâ€™, where simultaneously-performed parts accent the divisions of the same equally divided bar in different symmetrical combinations (for example, [1-2/3-4/5-6] set against [1-2-3/4-5-6] a favourite device of the composer Brahms); (e) where simultaneously-performed parts have differently accented divisions of the same bar, but where the dominant part shifts the conventional accents off the beat while the subordinate or accompanying part provides the basic regular beat (for example, [1-2-3/4-5-6/7-8] against [1-2-3-4/5-6-7-8]); (f) where simultaneously-performed parts have different accentual patterns, or metres, for example when three bars of four beats are set against four bars of three beats, the cycle recurring every twelve beats; (g) â€˜polymetric musicâ€™, where simultaneously-performed parts have different speeds or tempos, with the result that they do not converge on a common beat.
To the nonspecialist, such technicalities may seem abstruse. But in performance, the effect of polyrhythm is easily perceived and is exhilarating and stimulating, the brain simultaneously reacting to the â€˜reassuranceâ€™ of a perceived, regular rhythm, and to the â€˜surpriseâ€™ of rhythmic variants or contrasts being performed at the same instant. In African folk music in particular, and in musics such as jazz which make extensive use of syncopation, polyrhythm is a major intellectual and emotional resource. KMcL SSt
Further reading Simha Arom, African Polyphony and Polyrhythm; , Curt Sachs, Rhythm and Tempo.